On-load tap changers have been in worldwide use in large numbers for many years for uninterrupted switching between different winding taps of tapped transformers. So-called reactor changers, which are widespread particularly in North America, comprise a switching reactance enabling a slow, continuous switching. On-load tap changers according to the resistance rapid-switching principle usually consist of a selector for power-free selection of the respective winding tap of the tapped transformer which is to be switched over to and a load changeover switch for the actual switching from the previous to the new, preselected winding tap. The load changeover switch for that purpose usually comprises switches and resistance contacts. The switches then serve for direct connection of the respective winding tap with the load diverter and the resistance contacts for temporary connection, i.e. bridging over by one or more switch-over resistances. However, developments in recent years have led away from load changeover switches with mechanical switches in insulating oil. Instead, increasing use is made of vacuum switching cells as switching elements.
An on-load tap changer with vacuum interrupters is known from, for example, DE 10 2009 043 171 [U.S. Pat. No. 9,030,175]. Here, a load changeover switch carries a drive shaft that is drivable by a force-storing unit with at least one cam disc. The cam disc has a plurality of cams, wherein two cams arranged on the cam disc at the end have a contour that departs from a circular shape in the manner of lobes at which a respective roller connected with a vacuum interrupter by a rocker lever is guided under maintained contact, which roller tracks the profiled contour of the respective cam.
Due to the constructional configuration of this on-load tap changer this requires a spring force-storing unit for abrupt switching by the contact system. Force-storing units known from the prior art are pulled up, i.e. stressed, by a drive shaft at the start of each actuation of the on-load tap changer. The known force-storing units essentially consist of a pull-up carriage and a jump carriage between which energy storage springs as force-storing unit are arranged.
Force-storing units of that kind are evident from, for example, DE 198 55 860 and DE 28 06 282 [GB 2014794]. Despite these force-storing units used over decades there is repeated failure of these devices. Since the on-load tap changer is in use over a lengthy period of time the compression or tension springs repeatedly break and thus prevent switching. Moreover, it can happen that a carriage does not reach the end position, the switching shaft thus does not completely rotate and the switches do not reach the end position thereof. In the worst case this can lead to destruction of the entire tapped transformer.
By comparison with the prior art, the latest on-load tap changer models of the applicant do not have a mechanical force-storing unit for performance of switching processes. Actuation takes place directly by an electric drive. In the event of sudden failure of the energy supply for such a drive during a switching process, however, critical settings in the on-load tap changer can arise. These are, in particular, shortly before closing or after opening of a switch. In that case, it is possible, for example, for welding of the contacts inside the vacuum interrupter to occur.